enhance: tweak ToFileSize for readability

[NathanBaulch]

The new file size format has too many significant digits for good readability, plus thousands separators would help for the raw byte count.
Also, is now a good time to start adding some unit tests?

Before: 15.103 KB (15465 B)
After:  15.1 KB (15,465 B)

Before: 491.268 KB (503058 B)
After:  491 KB (503,058 B)

Before: 1.465 MB (1536332 B)
After:  1.46 MB (1,536,332 B)

[love-linger]

I've pushed my implementation for this request which based on current culture

[NathanBaulch]

Your implementation doesn't address this PR - too many digits in the formatted file size.
For example it now shows 491.268 KB when 491 KB is all the user needs to see. G3 is much better than N3 in this situation.